Subclover (Trifolium subterraneum L.) is a much valued annual legume forage used in ruminant production systems, particularly in drylands. It is particularly suited to grazing and can be cut for silage and hay.

Morphological description

Subclover is a prostrate annual clover reaching a height of 10-15 cm. It is taprooted with many lateral secondary roots. The stems are hairy, about 1-2 m long, trailing, but not rooting. The leaves are alternate, trifoliolate, cordate, borne on petioles. The flowers are whitish in colour, and are clustered in small seed-heads of 6-8 flowers. After self-fertilization, the fruit peduncle bends down and the developing seeds are buried until they germinate in autumn (FAO, 2017). The seeds are characterized by a high level of hardseededness (40 to 70% of hard seeds), a trait that depend on cultivar, on flowering conditions and grazing management (see Forage management below) (Quinlivan et al., 1971; Steiner et al., 1986; Stockdale, 2005).

Subclover originated from the Mediterranean basin and from the Middle-East. It is best suited to regions with wet and mild winters, and warm and dry summers. Subclover is particularly suited to semi-arid regions thanks to its short winter-growing habit. Trifolium subterraneum was introduced into Australia in the 19th century for ruminant production systems. It is now the most widespread legume in southern Australian pastures (Li et al., 1992; Stockdale, 1992b). Subclover is sown over 8 million hectares (ha) in Western Australia (WA) and 29 million ha across Southern Australia (Nichols, 2017). It was later introduced into the USA (Duke, 1981). In drylands of New Zealand, in the USA and in Australia, it is used in mixed stands of legume and grasses (Moot, 2012; Maxwell et al., 2016; Steiner et al., 1986; Curll et al., 1983). Subclover has also been selected and cultivated in Northern Mediterranean countries such as Spain, Portugal, Italy and France (Masson, 1997; Molle et al., 2008).

In Northern Africa and Western Asia, Trifolium subterraneum grows naturally on humid soils of open plains and in mountains, from sea level up to an altitude of 1800 m (Ecocrop, 2017). Subclover can grow in places where annual rainfall is above 400 mm. It is adapted to neutral and acidic soils. It does better on light, well-drained soils (Duke, 1981). Subclover can be cultivated in rainfed and irrigated orchards (Peaceful Valley, 1988 cited by UC SAREP, 2006).

Forage management

Yield

Management

Subclover is suited to permanent and semi-permanent pastures and to crop rotations, with at least 2 years between each crop (Nichols, 2007). It should be sown on a well-prepared seedbed. Establishment is slow in autumn, but growth period starts early in late winter and is particularly high in spring. Depending on ecotype or cultivar, flowering starts in March-April and ends in May-June in the Northern Hemisphere, and starts in September-October and ends in November-December in the Southern Hemisphere. The grazing period corresponds to spring and summer. Subclover withstands heavy grazing as its seeds are buried below ground and allow further self regeneration of the stand. Grazing during early flowering period promotes flower and seed production (Delagarde et al., 1993; Steiner et al., 1986; Rossiter, 1961). Heavy grazing prevents subclover from being shaded out by more erect plants (Nichols, 2007).

The regeneration of subclover stands depends on seed production, weight of seeds, and hardseededness, the first two factors being controlled through adequate grazing management (see above). Hardseededness, which prevents early germination in summer, is partially removed by high summer temperatures leading to mechanical breaking of the seed tegument (Stockdale, 2005; Steiner et al., 1986).

Environmental impact

Cover crop and fire control

Subclover is used for controlling fires in Mediterranean forests (strips to be grazed between forests) or as green winter culture in viticulture or arboriculture to avoid erosion (Masson, 1997; Molle et al., 2008). It can help to reduce soil erosion when combined to vegetable crops (Stirzaker et al., 1993).

Soil improver

Subclover is an N-fixing legume that has been shown to benefit to neighbouring grasses or subsequent cereals (Glatzle, 1989 cited by UC SAREP, 2006). Assessed as living mulch for tomato and lettuce production, subclover resulted in greater bulk density, greater water content and lower air porosity of the soil. Tomato yields were significantly greater with subclover (Stirzaker et al., 1993).

Nutritional aspects

Nutritional attributes

Subclover is a very good quality forage, of similar nutritional quality to berseem clover, red clover and white clover, during all the growing phases (Brink et al., 1994), or to burr medic and sulla (Sölter et al., 2007). As other typical grassland clovers, leafy, immature, vegetative subclover is characterized by a high protein concentration (16-30% DM), low fibre concentrations (NDF 25-35% DM, ADF 15-25% DM and lignin 2-4% DM) (Stockdale, 1992a; Stockdale, 1992b). Proportion of hemicellulose, cellulose and lignin in cell wall is typical of that of other clover species (Brink et al., 1994; Li et al., 1992). At vegetative stage, when grazed every 2 weeks, subclover leaflets have a very high protein concentration: 25 to 35% DM, whereas petioles and stem have a lower and quite similar protein concentration (13-22% DM) (Mulholland et al., 1996). After the flowering period, subclover protein concentration is lower, but still very high in leaflets (25%, 11%, 18% DM in leaflets, petioles or stem, and burrs, respectively). Vegetative leaflets have very low ADF and ADL concentrations (10-15% and 20-40% DM, respectively), while petioles and stem have an ADF content of approximately 16-20% DM during vegetative stage, and 22-26% DM at more mature stage (Mulholland et al., 1996). As subclover is an annual species, its nutritive value rapidly decreases at late maturity (mid or end of spring) when the proportion of pods increases, and before death (Mulholland et al., 1996). Dry mature residues of subclover during summer contain 12 to 21% protein, 50-60% NDF, 30-40% ADF and 8-12% lignin (DM), respectively, with an in vitro DMD ranging from 40 to 55% (Li et al., 1992; Ru et al., 2001).

Oestrogens

Some subclover varieties contain oestrogens. Particularly, early subclover cultivars contained formononetin, a deleterious phyto-oestrogenic isoflavone that has short to long-term negative effects on reproduction in sheep (Nichols, 2017; Reed, 2016; Pace et al., 2011). Newer cultivars selected to have low formononetin concentration (< 0.2% DM) have no detrimental effect on sheep reproductive performance (Nichols, 2017; Reed, 2016; Pace et al., 2011). When subclover constitutes a high proportion of ewe diets, breeding troubles may occur if subclover is grazed during its early stages of growth, since the formononetin oestrogen is likely to be present only at this stage (Nichols, 2017; Nichols, 2007). Subclover also contains the oestrogenic compounds genistein and biochanin A, but these compounds are of less concern (Nichols, 2017).

Ruminants

Nutritive value

DM digestibility of vegetative subclover is as high as that of white clover (> 80%) (Bolland et al., 2001). In vitro digestibility of OM in DM has also been found equal to that of burr medic or sulla during several periods of the year (Sölter et al., 2007). In vitro DMD of all plant parts is high at vegetative stage (70-80%). After cessation of flowering, DMD of leaflets, petioles, stem and burrs average 70%, 65%, 60% and 45%, respectively (Mulholland et al., 1996; Ru et al., 1999; Ru et al., 2000). Dry mature residues of subclover during summer had an in vitro DMD ranging from 40 to 55% (Li et al., 1992; Ru et al., 2001). Dry herbage feeding value over summer was reported to be less than maintenance value, often < 50% in vitro digestibility (Nichols, 2007). However, animals may be able to obtain sufficient energy and protein by digging up seed burrs that are rich in nitrogen, phosphorus, potassium, magnesium, zinc and copper, but poor in calcium and manganese (Ru et al., 2001).

Dairy cows

Vegetative fresh subclover can be an excellent feed for lactating dairy cows (Stockdale, 1992b; Wales et al., 2003). When fed alone, the intake level can reach 18.5 kg DM/d indoors and 21 kg DM/d at grazing for cows weighing 450 kg, which is a high intake level of 4 to 5% of BW (Stockdale, 1992b), leading to a production of 28 kg of milk/d in early lactation, with no concentrate supplementation. Extra milk produced per each additional kg DM eaten is 1.4 kg and 1.0 kg in early and late lactation, respectively, both indoors and at grazing, which clearly indicates high energy and protein values (Stockdale, 1992b). When pure subclover is grazed at low pasture allowance, milk production response to 5 kg of grain supplement is high (1.0 kg of milk per kg DM of concentrate), while providing an additional 1.7 kg of straw had no effect on milk production, milk fat concentration, and ruminal fermentations, although rumination time increased (Wales et al., 2003).

In vivo DMD of subclover fed alone to dairy cows indoors ranged from 0.77 to 0.84, which was greater than in vivo DMD measured simultaneously on sheep (0.75 to 0.79) (Stockdale, 1992a). Vegetative subclover is highly ruminally degradable, with a very rapidly degradable fraction of 35-45%, and a potentially degradable fraction of 47-55% for DM. The corresponding figure for nitrogen is 20-50% and 45-75% for rapidly and potentially degradable fractions, respectively. Nitrogen seems at least as degradable in stem as in leaves (Stockdale, 1992a).

Beef cows

No information found (as of 2017).

Sheep

Nutritive value of subclover and animal response has been widely studied in sheep, in terms of ruminal digestion and degradability, but also intake, weight gain and wool growth. Sheep show quite similar preference for subclover than for other annual or perennial clovers, and subclover is always well accepted and selected at grazing (Maxwell et al., 2016).

Merino wethers weighing 45 kg can eat 1080 g DM/d dried subclover (2.4% of BW), with an in vivo OMD of 78% (Hogan, 1973). Voluntary intake of dry subclover by sheep decreased on average from 1050 g DM/d to 900 g DM/d (i.e. -15%) between the 50%-flowering stage and a later harvest 30 days after (Nandra et al., 1998; Li et al., 1992).

Protein ruminal degradability measured in sheep from the nylon bag technique was found to vary from 51% to 64% (Stockdale, 1992a). Rapidly degradable fraction varied from 23 to 31% and potentially degradable fraction ranged from 49 to 64% for DM, according to cultivar and harvest date (Nandra et al., 1998). Nitrogen appeared less rapidly degradable than DM (16-20%), while potentially degradable nitrogen fraction is high (60-72%). When mature, both DM and nitrogen ruminal degradabilities are lower (Li et al., 1992).

Goats

No information found (as of 2017).

Poultry

Subclover is not used as an ingredient in poultry feed formulation, but it can be used by free-range poultry on pure or mixed pastures. In Brazil, subclover improved growth and feed conversion in free-range broilers by stimulating the intake of basal diet, even though the intake of subclover remained low (Ponte et al., 2008b). When basal diet was restricted, consumption of subterranean clover pasture significantly increased weight gain (Ponte et al., 2008a). The consumption of fresh subclover forage can be associated with positive sensory attributes (Ponte et al., 2008b).

Rabbits

Fresh subclover is particularly palatable for rabbits. For example, it is selectively grazed by wild rabbits in Australian pastures improved with a mixture of graminaceous and legumes forages (Croft et al., 2002). In agreement with this observation, in a study conducted to feed white-tailed deer in Louisiana (USA) with various forage plants including subclover, it was necessary to protect the experimental plots from wild rabbits with a special wiremesh (Johnson et al., 1993). Fresh subclover at pre-flowering stage was recommended as a standard forage for Angora rabbits production in Germany. The voluntary green feed intake of Angora rabbits was the greatest with subclover, compared to other forages such as meadow grass, rape or red clover (Schlolaut, 1987). In a Tunisian study of reproduction of rabbit does managed in groups with possibility of grazing, the results obtained with subclover were similar to those obtained with the traditional oat-vetch mixture (Kennou et al., 1990).

Subclover forage may therefore be considered as suitable for rabbit feeding, and can be used mainly as a source of protein. However, it should be noted that its protein is deficient in sulphur-containing amino acids (only 40-45% of requirements) and that is has a low phosphorus content (Lebas, 2013).

Horses and donkeys

When comparing the forage acceptability to horses of several grasses and clovers in Southern USA, crimson, berseem and subterranean clover were found to be highly palatable (McCann et al., 1991).